The clashes in Omran province killed 40 Huthi rebels and their allies and left 20 dead in the ranks of the Hashid tribe, the sources said.

The violence in Omran dates back to January 5 when Huthis tried to seize Hashid strongholds.

"The fighting, the heaviest since the clashes broke out in Omran, erupted at dawn on Friday in Wadi Khiwan and Wadi Danan and in other areas of the Huth and Isha districts," a tribal elder said.

Gunmen on both sides fired machine-guns and mortar rounds, he added.

Another source close to the powerful Al-Ahmar tribal grouping to which the Hashid belong said the tribes had "mobilised thousands of gunmen over the past few days" to confront the Huthis.

The source blamed the Huthis for launching an offensive on Tuesday, pounding the region with rockets and mortar rounds "in a bid to advance on the Huth district and Al-Khomri," Al-Ahmar bastions.

"Tribesmen launched a counter-offensive and were able to push back the Huthis and today's fighting was the fiercest," the source said.

Hashid tribesmen recaptured ground lost to the Huthis earlier in the week, he said, adding however that the fighting was still underway late Friday.

The rebels have been pushing out from their stronghold in the mountains of the far north to other areas nearer the capital to lay a stake to their own autonomous unit in a promised federal Yemen, political sources say.

But their fighters, know as Huthis from the name of the rebels' leading family, have faced stiff resistance from pro-government Zaidi tribes, as well as from Sunni hardliners who have established religious schools in parts of the north.

President Abdrabuh Mansur Hadi has pledged that Yemen will adopt a federal constitution to tackle the grievances of its disparate regions.

But at a ceremony last Saturday to mark the conclusion of a troubled 10-month national dialogue, he put off any decision on the thorny issue of how many component units it will have, promising that a special commission will decide.

The prospects of a federal Yemen, originally mooted as a solution to the grievances of the formerly independent south where secessionist violence has been on the rise, has spawned demands for autonomy from other discontented regions, including the rebel-held far north.